Make McpJson public to allow flexible protocol development

Author: parnurzeal

src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/Protocol.kt

@@ -26,7 +26,7 @@ public typealias ProgressCallback = (Progress) -> Unit
 
 @OptIn(ExperimentalSerializationApi::class)
 @PublishedApi
-internal val McpJson: Json by lazy {
+public val McpJson: Json by lazy {
     Json {
         ignoreUnknownKeys = true
         encodeDefaults = true